Modeling the Ash Distribution of a Yellowstone Supereruption (2014)

August 27, 2014

The topic of Yellowstone supereruptions (ones producing greater than one thousand cubic
kilometers
of volcanic debris) generates much interest, but also occasional confusion. New computer models
can
help
clarify the reality of eruption impacts, and provide insight on past eruptions. In August 2014, USGS
scientists Larry Mastin and Jacob Lowenstern, and National Science Foundation researcher Alexa
Van
Eaton
published research on where volcanic ash would fall if a Yellowstone supereruption were to occur
today
(with present-day weather patterns); the research models an eruption similar to the caldera-
forming
event
that occurred 640,000 years ago. Yellowstone is an obvious target for ash distribution studies, as it
provides an opportunity to understand very large eruptions that generate umbrella clouds, which
spread
radially in the atmosphere (see last image below). The distribution of deposits from ancient
Yellowstone
eruptions can be compared with the output from the computer models.

Why did you write this paper?

At YVO, we are constantly asked questions like "how much ash would I get if Yellowstone had
another
supereruption." Instead of stating "we don't know" or "it depends on the winds," we decided to take
advantage of an ash-dispersion modeling program, co-developed by Larry Mastin who works at the
USGS
Cascades Volcano Observatory. He spent many years working with colleagues to develop computer
models
that predict ash fallout from volcanic eruptions, and he decided to try applying the new model to
Yellowstone.

What about the maps showing Yellowstone deposits across the U.S.? Don't they indicate
where
ash
will go?

No. They show an outline of the area where ash has been found from the previous big
eruptions
at
Yellowstone (640,000, 1,300,000, and 2,100,000 years ago). However, they do not provide
information on
the original ash thickness at any particular location. One reason we don't know the thicknesses
from
the
previous eruptions is because the ash deposits are eroded and rapidly re-distributed by rain, rivers,
and
wind in the years following the original eruptions. The very thin deposits far away from the volcanic
source
are not preserved at all in the geologic record.

Lack of reliable information left the door open to speculation and fanciful depictions of the effects
of
supereruptions, which are easily found on the Internet. Results of the new study show that ash
accumulation, while widespread and substantial, is far less than in most of these "doomsday"
scenarios.

What is new and significant about this study?

Models have been used for decades to forecast ashfall during eruptions. But only in recent
years
have
tephra models like Ash3d been developed that use a 3-D, time-changing wind field, enabling us to
model
eruptions that last weeks and spread ash across an entire continent. These features, plus the
development
of a method for calculating growth of an umbrella cloud, have made it possible to simulate
eruptions
of
this scale.

Did you learn anything new of scientific interest through this modeling?

Yes, we learned that supereruptions distribute ash in a fundamentally different pattern than
smaller
eruptions by creating an umbrella cloud that can push ash more than a thousand kilometers
upwind.
The
mapped pattern of ash deposition from weaker eruptions looks roughly like a fan, spreading
downwind
from the volcano; while that from a supereruption looks more like a bull's eye, centered on the
volcano. A
powerfully spreading umbrella cloud means that ash dispersal is much less affected by atmospheric
winds.

What's happening geologically at Yellowstone now?

Seismicity and ground deformation are within historical norms. The caldera started moving up
this
year after about four years of slow subsidence. Earthquakes were more abundant early in 2014
than
in mid
2014, especially in the area near the Norris Geyser Basin.

Is there any evidence that Yellowstone will erupt soon?

No. Yellowstone is behaving as it has for the past 140 years. And geological evidence
indicates
that
similar or higher rates of earthquakes, ground uplift and steam explosions were experienced at
Yellowstone over much of the past ~10,000 years. Odds are very high that Yellowstone will be
eruption-
free for the coming centuries.

If Yellowstone erupts, will it be the "big one" modeled in this recent article?

Almost certainly not. The past 20 eruptions at Yellowstone have been lava flows with no
significant
amounts of ash fall outside of Yellowstone. The past 60-80 eruptions would have had little
regional
(or
continental) impact.

How will you know if an eruption is beginning?

Yellowstone hasn't erupted for 70,000 years, so it's going to take some impressive
earthquakes
and
ground uplift to get things started. Besides intense earthquake swarms (with many earthquakes
above M4
or M5) we expect rapid and notable uplift around the caldera (possibly tens of inches per year).
Finally,
rising magma will cause explosions from the boiling-temperature geothermal reservoirs. Even with
explosions, earthquakes, and notable ground uplift, the most likely volcanic eruptions would be the
type
that would have minimal affect outside the park itself.

Who is YVO?

Besides the USGS, YVO includes staff from two universities (Utah and Wyoming), three state
geological
surveys (Wyoming, Montana and Idaho), Yellowstone National Park, and UNAVCO, a non-profit that
installs
and maintains geophysical equipment funded primarily through the National Science Foundation.
Of
course, we also have the resources of all other US volcano observatories available to help (Alaska,
Hawaiian, Cascades and California volcano observatories). We are in frequent contact with other
scientists
throughout the US and all over the world. Our data is distributed globally, so anyone with resources
and
experience can assist in monitoring.

Should I prepare for an eruption?

Regardless of where you live, it's always a good idea to be prepared for emergencies,
including
earthquakes, tornadoes, chemical spills, and other random events. Emergency experts recommend
keeping
extra supplies on hand and creating a family emergency communication plan. Beyond that, there is
no
reason to specifically prepare for an eruptive event at Yellowstone, which remains very unlikely.